The present invention relates to a method for manufacturing a spark plug and a caulking metallic mold for use with the method.
The metal shell of the spark plug is typically composed of iron material such as carbon steel. For the corrosion prevention, a metal plating layer of zinc or nickel is coated on the surface of the metal shell, or a chromate film may be further applied on the surface formed with the metal plating layer. Of these surface treatments, the chromate film containing chromium (VI) (hereinafter referred to as a chromium (VI) film) as chromium constituent has a particularly excellent corrosion resistance, and is suitably employed for the sparkplug. However, the chromium (VI) film contains chromium (VI) as its chromium constituent, and tends to be gradually shunned in these days when there is a rising tide of environmental protection. Therefore, the chromium (VI) film is examined to be done away with in the future.
Thus, the chromate film containing little chromium (VI), namely, the chromate film containing chromium (III) as most of chromium constituent (hereinafter referred to as a chromium (III) film) has been developed from relatively early on. This chromate film can be formed in a treatment bath having a relatively small content of chromium (VI), or may be formed in a treatment bath not containing chromium (VI) at all.
The chromium (III) film as described above was difficult to form in large thickness, and to attain a more excellent anticorrosion than the chromate (VI) film. However, with the development of treatment bath, the thickness of chromate film could be increased, and the excellent anticorrosion obtained. Accordingly, the chromate (III) film tends to be suitably used, with the chromate (VI) film, to prevent corrosion in the metal shell of the spark plug.
Generally, a method for attaching the metal shell of the spark plug to the outside of an insulator inserted inside and having a central electrode disposed at a top end of the metal shell involves caulking and fixing a rear end periphery (portion to be caulked) of the cylindrical metal shell that is curved toward the outer circumferential face of the insulator.
However, if the metal shell having the chromate (III) film formed on its surface is employed, various dimensions of the metal shell often deviate from the tolerance after caulking. The deviation of various dimensions from the tolerance (hereinafter referred to as a dimensional deviation) may be confirmed even when other surface treatment including applying the chromate (VI) film is made on the metal shell, but was especially conspicuous when the chromate (III) film was formed. This dimensional deviation impedes the sufficient effect of caulking. Particularly, if the dimensional deviation of the opposite side of the tool-engaging portion or the caulking height is excessive, the bulk density of talc packed between the inner circumferential face of the metal shell and the insulator or the air-tightness of the spark plug itself is unfavorably decreased. Thus, to suppress this dimensional deviation, the caulking metallic mold useful in caulking and fixing the metal shell to the insulator has a deep compression scroll of the portion to be caulked. By deepening the compression scroll of the portion to be caulked, the opposite side size of the tool-engaging portion is kept from expanding more easily.
However, the above caulking metallic mold is effective at the early time of use, but less effective as the caulking of the metal shell is repeated, resulting in remarkable dimensional deviation of the metal shell after caulking. This dimensional deviation was especially conspicuous in forming a zinc plating layer as the substrate metal plating layer on the metal shell, and forming the chromate (III) film thereon, but tended to occur when other surface treatments were made.
It is an object of the present invention to provide a method for manufacturing a spark plug and a caulking metallic mold for use with the method in which the deviation of various dimensions of the metal shell after caulking is suppressed within a tolerance even though the metal shell is caulked and fixed repeatedly to the insulator.
To achieve the above object, according to the present invention, there is provided a method for manufacturing a spark plug in which a portion to be caulked of a cylindrical metal shell having a tool-engaging portion to be attached on an engine is caulked and fixed around an outer circumferential face of an insulator extending axially and inserted into the metal shell, characterized in that a caulking metallic mold for caulking and fixing the portion to be caulked is formed with a hard carbon film mainly composed of amorphous carbon phase on a surface contact and sliding with the portion to be caulked of the metal shell.
Further, there is provided a caulking metallic mold for use with the method for manufacturing the spark plug, in which the caulking metallic mold is employed to caulk and fix a portion to be caulked of a cylindrical metal shell having a tool-engaging portion to be attached on an engine around an outer circumferential face of an insulator extending axially and inserted into the metal shell, characterized in that the caulking metallic mold has a hard carbon film mainly composed of amorphous carbon phase formed on a surface contact and/or sliding with the portion to be caulked of the metal shell. The surface contains a first layer containing chromium or titanium and a second layer containing silicon or germanium, and the hard carbon film is formed on the second layer. By forming the hard carbon film on an intermediate layer (the first layer and the second layer) that is a double layer structure, the adhering strength of the hard carbon film to a main body can be heightened, and the hard carbon film can be prevented from peeling in a caulking process for a long period of time. The caulking metallic mold contains the surfaces for contacting and/or sliding on both upper and lower surfaces, and each of the surfaces for contacting and/or sliding can be used to the caulking process if the upper and lower surfaces are turned around. Since the hard carbon film (and the intermediate layer) can be formed on the upper and lower surfaces, a cost is not especially taken for composing such a structure. Accordingly, if the caulking metallic mold is made reversible, a cost-up is not taken for producing the metallic mold, but one metal mold can be used twice by turning it around, and the cost for the metal mold may be saved cheaply.
The deviation of various dimensions of the metal shell after caulking arises because undesired stress is applied on the metal shell at the time of caulking to induce an undesired deformation of the metal shell. To reduce the undesired stress, it is effective to increase the sliding property between a surface of the caulking metallic mold contact and sliding with the metal shell and the metal shell. Thus, the present inventors, as a result of minute examination, have found that if the caulking metallic mold having a hard carbon film mainly composed of amorphous carbon phase made on the surface of the caulking metallic mold contact and sliding with the portion to be caulked of the metal shell is employed, the sliding at the time of caulking is excellently conducted, and the deviation of various dimensions of the metal shell after caulking can be effectively suppressed, and have completed this invention.
As used in this specification, the xe2x80x9chard carbon film mainly composed of amorphous carbon phasexe2x80x9d means that the skeleton structure of carbon mainly constituting the film is amorphous, and its Vickers hardness is 1500 kg/mm2 or greater. The preferable range of the thickness of the hard carbon film is 0.6 to 1.2 xcexcm. If being less than 0.6 xcexcm, an effect by forming the hard carbon film is less, while being more than 1.2 xcexcm, an adhering strength of the hard single film itself decreases, and the film is easy to peel. The hardness of film is measured by, for example, a dynamic micro hardness tester. The hard carbon film, including many diamond bonds of carbon in the bond making up the skeleton structure of amorphous carbon, is referred to as a DLC (Diamond Like Carbon) film, with the hardness similar to that of diamond. Therefore, the hard carbon film represented by the DLC film has an especially small friction coefficient, and has the effect of increasing the sliding property with other members. In this invention, the sliding property with the portion to be caulked of the metal shell is increased by forming the hard carbon film mainly composed of amorphous carbon phase represented by this DLC film on the caulking metallic mold. As used in this specification, xe2x80x9cchieflyxe2x80x9d or xe2x80x9cmainlyxe2x80x9d means involving the greatest content (mass %) in the fabric of interest.
Also, in this invention, the metal shell is plated with zinc or nickel at least on an outer circumferential face of the portion to be caulked, and further treated with chromate on the surface, or only plated with nickel. These surface treatments are typically performed for the metal shell of the spark plug. In this invention, when caulking and fixing the metal shell subjected to the typical surface treatment, the deviation of various dimensions from the tolerance can be suppressed, resulting in significant industrial effect.
The chromate film made on the surface of metal shell may be either chromate (VI) film or chromate (III) film. That is, the deviation of various dimensions of the metal shell in forming the chromate (III) film is especially conspicuous, and owing to the invention, the dimensional deviation can be suppressed effectively. However, when the chromate (VI) film is formed, the invention can be also applied effectively (i.e., the dimensional deviation can be further suppressed). Further, the invention is effective when the metal shell is formed with the chromate film as well as when it is only plated with nickel.
Also, if the conventional caulking metallic mold is employed in forming the metal plating and/or chromate film on the surface of the metal shell, as described above, there was a tendency that the plating defect such flaking or roughness arises more severely with greater use frequency of the caulking metallic mold. However, when the caulking metallic mold of the invention is used, there is the effect that the plating flaking or roughness is less likely to arise as compared with when the conventional caulking metallic mold is used, even if the use frequency of the caulking metallic mold is increased (even if the caulking is repeated many times). Specifically, when the caulking metallic mold of the invention is used, there is no plating defect at the caulked portion of the metal shell, even if used tenfold or more, unlike the conventional caulking metallic mold.
When the metal shell is formed with a chromate film, the chromate film having a film thickness of 0.2 to 0.5 xcexcm and containing chromium (III) at 95 mass % or more of chromium constituent may be made at least on the outer circumferential face of the portion to be caulked. The chromate film containing chromium (III) at 95 mass % or more of chromium constituent referred to as a chromate (III) film in broad sense) has a content of chromium (VI) of less than 5 mass %, and a significant effect on the environmental measures is expected in employing the chromate film. It is desirable that the chromate film does not contain substantially chromium (VI) in the respect of environmental protection. Since this chromate (III) film involves the especially conspicuous deviation of various dimensions of the metal shell in caulking, as previously described, the effect of the invention can be further expected.
In consideration of the service condition of the spark plug, the film thickness of the chromate (III) film made on the metal shell is preferably set at a value from 0.2 to 0.5 xcexcm. If the film thickness is above 0.2 xcexcm, the durability of the chromate (III) film can be fully secured even in the service conditions specific to the spark plug which are subject to the rising temperature and the attack by acid. On one hand, if the film thickness is beyond 0.5 xcexcm, there occurs a crack on the film in caulking, or an exfoliation of the film, resulting in lower durability. The film thickness of chromate (III) film is preferably set in a range from 0.3 to 0.5 xcexcm.
However, in the chromate (III) film having the above film thickness, there is a tendency that the deviation of various dimensions especially arises at the time of caulking. This is considered due to the fact that the chromate (III) film is formed through the wet process, the water content in the film is relatively higher, and the water content is excessively distributed particularly on the surface of the chromate film having the film thickness as mentioned above. Namely, due to this water content, an undesirable adsorptive force is exerted on the caulking metallic mold that is slid with the metal shell, impairing the sliding property between them, and causing the dimensional deviation of the metal shell.
With this invention, if the hard carbon film is formed on the caulking metallic mold, the chromate (III) film on the metal shell is prevented from being adsorbed to the caulking metallic mold due to the water content, providing the excellent sliding property. And the deviation of various dimensions in caulking can be suppressed.
When forming the zinc plating layer on the surface of the metal shell, and then forming the chromate (III) film thereon, the dimensional deviation is especially remarkable. This is considered due to the fact that zinc and chromium constituents adhere to the caulking metallic mold by repeating the caulking, hampering the sliding property between the caulking metallic mold and the metal shell. In practice, these adhering constituents are observed on the surface of the caulking metallic mold after use. This invention also exhibits the effect in this situation. This is considered due to the fact that the hard carbon film prevents zinc or chromium from adhering to the caulking metallic mold, maintaining the excellent sliding property with the metal shell.